1. Field of the Invention
The invention relates to an optical signal rejection filter which can be used, in particular, to reject the even frequencies and retain the odd frequencies of an optical signal consisting of the various harmonics of a fundamental frequency. The invention also relates to an application of a filter of this type to ring interferometers.
The frequency filtering of optical signals raises problems which, depending on the techniques concerned, are resolved only by standard electronic methods.
The invention proposes a filter which can be used to eliminate even frequencies with certainty by associating a combination of optical devices with an electronic amplifying circuit.
This filter is more especially applicable to ring interferometers, more particularly, to interferometers having, in a branch of the ring-shaped waveguide of the interferometer, a phase modulation device which can detect, with high sensitivity, a phase shift between two counter-rotating waves that travel through the ring of the interferometer.
2. Description of the Prior Art
An interferometer of a known type has mainly a source of light energy generally consisting of a laser; an optical device consisting of either a number of mirrors or of an optic fiber wound on itself, this optical device forming a waveguide; a device for separating and mixing light and a device for the detection and processing of the signal detected.
Examples of descriptions of interferometers of this type will be found in the French patent applications:
No. 2 471 583 filed on Dec. 14th 1979, PA1 No. 2 566 133 filed on June 14th 1984.
It is known that, in these interferometers, there are two waves that come from the separating device and travel along the same optical path in opposite directions.
Disturbances or magnitudes to be measured are represented by phase shifts in the waves, and it is possible to measure a phase difference between two waves.
To measure low-amplitude disturbances causing minor phase differences, the French patent No. 2.471.583 has proposed a phase modulation, of alternately +.pi./2 radian and -.pi./2 radian, of the waves propagated in the ring.
Since a phase modulation is produced at one end of the fiber loop, one of the waves undergoes the modulation at the instant when it is produced, while the other wave undergoes the modulation with a delay equal to the propagation time in the fiber. This propagation time satisfies the following relationship:
t.sub.o = nl/c , where n is the equivalent refractive index of the fiber, 1 is the length of the fiber and c is the speed of light in vacuum. The natural frequency of the interferometer is (1/2t.sub.o), and represents the modulation frequency at which the two waves undergo their respective phase shifts in phase opposition. The phase shift between the two optical waves is therefore equal to the difference s(t)-s(t-t.sub.o), where S(t) is the signal applied to the phase modulator. It can therefore be seen that if the half period of the modulation signal is t.sub.o, the phase shift at the output of the interferometer is equal to twice the phase shift applied. This is the method used to create the slant that gives the functioning point of the interferometer.
To this phase shift, there is added a phase shift .DELTA..sym..phi..sub.o due to an effect to be measured. The signals can be used directly, and the component in sin.DELTA..phi..sub.o, carried by the odd harmonics, can be measured. However, there are even harmonics which are difficult to eliminate.
The filter of the invention, applied to an ring interferometer, can eliminate these even harmonics with certainty.